Magnetic Fluid

ABSTRACT

The present invention is in the field of fluids and the like comprising magnetic particles, such as ferromagnetic particles, anti-ferromagnetic particles, ferrimagnetic particles, synthetic magnetic particles, paramagnetic particles, superparamagnetic particles, such as magnetic fluids, a method of stabilizing magnetic particles, use of these fluids and functionalized particles. Such fluids have a large variety of applications, such as sealants, as a sensor, in biomedics, etc.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International ApplicationPCT/NL2014/050149, entitled “Magnetic Fluid”, to Ioniqa B. V., filed on12 Mar. 2014, which is a continuation to Netherlands Patent Applicationwith Serial No. 2010439, filed 12 Mar. 2013, and the specification andclaims thereof are incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

INCORPORATION BY REFERENCE OF MATERIAL SUBMITTED ON A COM-PACT DISC

Not Applicable.

COPYRIGHTED MATERIAL

Not Applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention (Technical Field)

The present invention is in the field of fluids and the like comprisingmagnetic particles, such as magnetic fluids, a method of stabilizingmagnetic particles, use of these fluids and functionalized particles.Such fluids have a large variety of applications, such as sealants, as asensor, in biomedics, etc.

2. Description of Related Art

Various patent documents and scientific documents recite fluidscomprising magnetic particles.

Magnetic Fluids are a class of smart materials that change theirproperties reversibly and fast (milliseconds) under presence of anexternal magnetic field. These fluids can show changes in apparentviscosity of several orders of magnitude when a magnetic field isapplied, such as a magnetic flux density in the order of around 1 T.Below two sub-classes are identified.

A ferrofluid relates to a liquid which becomes strongly magnetized inthe presence of a magnetic field. Typically ferrofluids are colloidalliquids made of nanoscale ferromagnetic, or ferrimagnetic, particlessuspended in a carrier fluid (usually an organic solvent or water).Typically each tiny particle is thoroughly coated with a surfactant toinhibit clumping. A disadvantage is that larger particles can be removedfrom an otherwise homogeneous colloidal mixture, forming a separateclump of magnetic dust e.g. when exposed to strong magnetic fields.

A difference between ferrofluids and magnetorheological fluids (MRfluids) is the size of the particles. The particles in a ferrofluidprimarily consist of nanoparticles which will not settle under normalconditions. MR fluid particles primarily consist of micrometer-scaleparticles which will settle over time because of the inherent densitydifference between the particle and its carrier fluid. These two fluidshave very different applications as a result.

A problem with many prior art magnetic fluids is that these are notsufficiently stable, especially over time. Therefore they can not bestored for a longer period. Many prior art fluids are also not stable athigher temperatures, e.g. temperatures of application and production ofthe fluid, and at low vapor pressure. Even further, they are also notstable when in use, in particular in an external magnetic field(gradient).

Production methods of prior art fluids are typically time consuming(slow), not efficient, e.g. in terms of energy and chemicals used, andare laborious.

A further problem is that many fluids comprising magnetic particles cannot withstand a high pressure difference, e.g. in sealing application.Also the fluids can not be used when a relative low external magneticfield (gradient) is present, as these fluids are not magnetic enough.Obtaining fluids can be complicated in view of stabilization procedures,a high temperature and inert atmosphere being required, and dialysisprocessing. Therewith application of the prior art fluids is limited.

It is noted that claims in various prior art documents with respect toe.g. densities obtained can not be obtained, sometimes already from aprinciple point.

An issue with prior art fluids is that they foam, especially whenagitated. The problem is often not recognized as such, for instancebecause fluids are only used on a lab scale. However, clearly for largeamounts of fluids to be produced foaming is an issue, e.g. in terms ofcontrollability.

Some prior art methods form magnetic particles at relatively high OH⁻concentrations. In order to control e.g. particle size and particlecomposition a relatively high temperature of 80-95° C. needs to be used.Without such extra measures e.g. magnetite can not be obtained.

It is noted that coating nanoparticles per se is known in the prior art,e.g. for protection thereof. A coated particle does not inherentlyrelate to particles that can be densified in a dispersion. Despitecoating of magnetic particles per se is known, such does typically notrelate to providing solutions to one or more of the present problems.

Incidentally some prior art recite methods which in view of the presentinvention could relate to forming a pre-dispersion having a lowconcentration of dispersed particles (at the most 5-7 vol. %). Forinstance DE 102 05 332 A1 recite a magnetic dispersion formed underargon atmosphere. Also Yu J-H et al. (J. Magnetism ad MagneticMaterials, vol. 304, September 2006, pp. e16-e18) recites an water baseddispersion formed under an inert atmosphere, with an oleic acid coatingwhich is considered unsuitable. Also Lin et al. (J. Coll. And InterfaceScience, Vol. 291, November 2005, pp. 411-420), recites a dispersionformed under an inert atmosphere, with a polyacrylic acid (PAA) coatinghaving a high molecular weight and long polymeric chains which isconsidered unsuitable. None of these documents recites a high densitydispersion. In fact the dispersions formed are considered unsuitable forhigh density dispersions.

The present invention therefore relates to a magnetic fluid, a method ofstabilizing magnetic particles and use magnetic fluids, which overcomesone or more of the above disadvantages, without jeopardizingfunctionality and advantages.

BRIEF SUMMARY OF THE INVENTION

The present invention relates in a first aspect to a method according toclaim 1 for forming a high density dispersion, a high density dispersionaccording to claim 7, use of a high density dispersion according toclaim 12, and a fluid comprising a high density dispersion according toclaim 13.

The density of the dispersion can be defined in terms of volume of themagnetic particles, with respect to a total volume of a dispersion, i.e.including solvent, additives, etc., and in a similar fashion in terms ofweight.

The present magnetic particles are present as very small particles,typically having a diameter of less than 10 μm. The particles thereforerelate to micro-particles, and more typically to nano-particles.

The present magnetic fluids are very stable with concentrations highenough to observe a spike when placed on a magnet. It is noted that along-term stability of the present fluid on a magnet or in a magneticfield gradient is not obvious at all. Very dilute fluids may survive astability test relatively easily, but for concentrated ones it is muchmore difficult.

It is noted that only upon thorough scientific investigations thepresent inventors have identified boundary conditions, such as chemicalsused, concentration of chemicals used, sequence of processing steps,required pH, and stages, that provide a solution to the problemsmentioned throughout the description and that provide the presentadvantages, of which at least some advantages are considered ratherunexpected. As a multitude of parameters was varied it was at forehandnot clear which of the parameters would contribute to the presentsolution and also not to what extent.

In a first stage a dispersion having a relatively low density is formed,i.e. concentration of the magnetic particles, is provided. Therelatively low density may in a next stage subsequently be increased byaddition of a weak organic acid, or a corresponding (well dissolving)salt, which acid interacts with the magnetic particles. The weak organicacid is provided directly after provision of the low density dispersion,i.e. without any intermittent step. Surprisingly a relatively lowconcentration of weak organic acid (or likewise a corresponding salt) isfound to be sufficient and, in strong contrast to prior synthesismethods, no high temperature, inert atmosphere or dialysis processing isneeded for the acid to interact with the magnetic particles. If apoly-acid is used, i.e. an acid comprising two or more hydrogen atomsthat can be released, a relatively lower concentration can be used. ThepKa of the weak acid is preferably in the order of 3 or higher. If apoly-acid is used, the pKa₁ is preferably in the order of 3 or higher.In other words the weak acid is at least partly dissociated at neutralor basic pH which corresponds to the reaction conditions. Theinteraction between the weak organic acid and the magnetic particlestakes place rather quickly. For specific examples, such as for citricacid and magnetite, the interaction is largely completed in less than 10seconds, especially upon (vigorously) stirring. It is however preferredto give the interaction more time to be completed as far as reasonablypossible, such as up to 5 minutes of time in the example.

It is noted that use of a weak acid in combination with a magneticparticles is quite atypical in the field and is in various instancesdiscouraged in the field.

During the interaction the pH is kept at a basic value, that is at anOH⁻ concentration of more than 10⁻⁷ mole/l. A slightly basic value ofthe pH is found to be sufficient. If the pH is too low, i.e. 7 or lower,or too high, the interaction is not as efficient as near pH 7 orslightly above; in view of the present invention it is regardedinsufficient and the present (very) high density dispersion can not beobtained.

Surprisingly the density of a dispersion comprising magnetic particlescan be increased for a wide range of magnetic particles, eitherparticles comprising only one or substantially one (chemical) element,such as iron, cobalt, manganese, etc., but also of particles having ametal ion and a counter ion, such as oxygen, boron and nitrogen. Alsocombinations, such as alloys, mixed particles, and the like areapplicable. In view of iron being abundantly available this element ispreferred, in particular magnetite (chemical formula Fe₃O₄) andmaghemite (Fe₂O₃, γ-Fe₂O₃). However, if e.g. higher magnetic densitiesare envisaged, other elements and combination can be applied as well.

A further advantage of the present method is that a size, or likewise anaverage size, of particles obtained is well controlled and can beobtained in a reproducible manner. Such is achieved by controlling,during synthesis of particles, a concentration of e.g. iron salts, anaddition rate of the base and an amount of the base, among others.

The present method does not need a further additional step, or at leastnot a further complicated step, such as high temperature, inertatmosphere or dialysis processing, such as addition or removal or anymanipulation with any surfactant or any anti-foaming measure. In factsurprisingly the present fluids do not foam at all.

Thereby the present invention provides a solution to one or more of theabove mentioned problems.

Advantages of the present description are detailed throughout thedescription.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates in a first aspect to forming magneticparticles in pre-dispersion according to claim 1. The method may beperformed in one reactor.

In an example of the present method the magnetic particles are one ormore of ferromagnetic particles, anti-ferromagnetic particles,ferrimagnetic particles, synthetic magnetic particles, paramagneticparticles, superparamagnetic particles, such as particles comprising oneor more of Fe, Co, Ni, Gd, Dy, Mn, Nd, Sm, and preferably one or more ofO, B, C, N, such as iron oxide, such as ferrite, such as magnetite, andmaghemite. The present magnetic particles align in an external magneticfield, especially in an external magnetic field with a gradient. Thepresent magnetic particles also relate to particles (and domainsthereof) being partially aligned.

In an example magnetic particles are (initially) present in an amount of10-30 wt. % relative to a total weight of the dispersion, that is arelatively low weight percentage.

In an example of the present method the carboxylic acid comprises a 4-20carbon atoms, for water based fluids 5-10 carbon atoms, more preferably6-8 carbon atoms, for oil based fluids 12-20 carbon atoms, morepreferably 15-18 carbon atoms. For hydrophilic ionic liquids (to someextent resembling water based fluids) smaller carboxylic acids arepreferred (5-10 carbon atoms), and likewise for non-hydrophilic orlipophilic ionic liquids (to some extent resembling oil based fluids)larger carboxylic acids are preferred (15-18 carbon atoms).

In an example of the present method the weak acid is a functionalizedacid, such as given in FIG. 1, and further identified in thedescription, wherein nε[1-10], preferably nε[2-6], and/or whereinmε[1-6], preferably mε[2-4], and/or wherein o (if present) ε[1-6],preferably oε[2-4], and/or wherein X is selected from F, Cl, Br, I,dicyanamide, bis(trifluoromethylsulphonyl)imide, preferably Cl, andcombinations thereof. In an example n=9, m=3, and X=Cl.

In an example of the present method the carboxylic acid comprises 1-4carboxylate groups, such as 2-3 carboxylate groups, such as citric acid,and a polycarboxylic acid. It is preferred to include formation ofbi-layers of the weak organic acids mentioned.

Likewise well dissolvable salts of the present acids may be used, suchas Na, K, and Li.

In an example of the present method the dispersion with magneticparticles is provided at a temperature between 15° C.-95° C., preferablybetween 20° C.-35° C., such as a 21° C.-25° C. In principle no heatingis required.

In terms of consumption and interaction it is preferred to provide theweak organic acid in an amount of 10⁻⁴-0.01 Mole acid/g magneticparticle, preferably in an amount of 5*10⁻⁴-5*10⁻³ Mole acid/g magneticparticle, more preferably in an amount of 8*10⁻⁴-2*10⁻³ Mole acid/gmagnetic particle, that is not a too low concentration, in order toobtain a largely maximized interaction between acid and particle, andnot too high, in view of consumption. In view of particle size andsurface area thereof, it is noted that for relatively larger particles,such as of 100 nm, relative lower concentrations may be used, i.e. at alower boundary of the ranges given. Likewise, for relatively smallerparticles, such as of 1 nm, relative higher concentrations may be used,i.e. at a higher boundary of the ranges given.

In an example of the present method interacting takes place during aperiod of less than 5 minutes, such as 1-4 minutes, and whereininteraction is supported by mixing. Such process times are considered tobe rather small. In comparison, prior art methods typically involve timeof 1-4 hours, i.e. a factor 60 (or more) longer. Such is also importantin terms of energy consumption, being extremely limited in the presentcase.

In an example of the present method after providing the weak organicacid the dispersion is washed one to four times, preferably with anorganic liquid, such as acetone, or a mixture of water and an organicliquid. Also a combination of washing steps may be used, e.g. a firststeps using water, a second step using an organic liquid, etc. Theorganic liquid reduces a stability of the dispersion. As a consequencethe particles settle and can be separated easily from e.g. asupernatant. Also organic liquid present can be evaporated easily.

In an example of the present method the particles are separated from thewashing liquid by a separation method, such as by sedimentation bysettling, centrifugation, and magnetic separation. An example of amagnetic separation is a use of one or more magnetic rods, makingseparation very effective. The magnetic rods may be entered into thedispersion, at that point optionally not being magnetic yet; then amagnetic field may be applied. The present magnetic particles are thenattracted by the magnet and can be separated easily, by turning off themagnetic field applied after removing the magnetic rods from thedispersion.

In an example of the present method the method is performed in air. Inother words, no extra measures have to be taken in order to carry outthe present method.

In an example of the present method after washing the dispersion thedispersion is re-dispersed in a solvent, such as water, oil, ionicliquid, such as a fluid in general.

In an example of the present method the dispersion is concentrated, suchas by separating the magnetic particles from a solvent, and providing asmaller amount of solvent to the separated magnetic particles.

In an example of the present method the weak organic acid is provided ata temperature between 15° C.-95° C., preferably between 20® C.-60° C.,such as a 21° C.-25° C.

After obtaining a pre-dispersion the magnetic particles obtained can bere-dispersed in a suitable solvent (or carrier), such as water, oil, anionic liquid, optionally comprising additives, and a high densitydispersion is formed. The dispersion may be further concentrated, e.g.by removing surplus solvent.

In a further example the particles may contain a further layer likestructure, and/or further coating. In a detailed example the presentmagnetic particles are provided in a first-layer with oleic acid, and ina second layer with poly isobutenyl succinic anhydride (PIBSA) andfinally dispersed in an oil, e.g. a polyalphaolefin oil. Then thedispersion may be mixed with a grease.

In a second aspect the present invention relates to a high densitydispersion comprising magnetic particles according to claim 7. Adispersion comprising a combination of the present magnetic particles isalso envisaged, such as particles having a first size, e.g. from 10-20nm, and particles having a second size, e.g. from 500-700 nm.

An advantage of the present dispersion it that it does not form a foamor foams.

A further advantage of the present dispersion is that they are stable inhigh external magnetic field, such as higher than 0.5 T. Experimentshave shown stable dispersion up to 1.3 T. Such is considered extremelystable. A use of the present dispersions is therefore extended over alarge range of technical applications, being not achievable with priorart dispersions.

The present dispersions are also very stable, e.g. over a long period oftime, such as typically more than six months, and in many cases muchlonger than a life time of an application, such as longer than years. Itis noted that many prior art dispersion are only stable for a shortperiod of time, e.g. days or weeks or less. Further the presentdispersions remain stable also in use, or likewise when used in anintermittent fashion. Also the present dispersions are not prone tooxidation or chemical degradation; as such no special precautions haveto be taken e.g. in order to preserve the dispersions.

The present dispersions take less energy to make, are faster (e.g. insealing, switching etc.), are better sealants under more extremeconditions, have a wider range of applications, etc.

In the present dispersion the magnetic particles are present in anamount >15 vol. % relative to a total volume of the dispersion,preferably between 20-50 vol. %, more preferably between 25-40 vol. %,such as between 30-35 vol. %, and/or in an amount >45 wt. % relative toa total weight of the dispersion, preferably >50 wt. %, morepreferably >60 wt. %, such as >80 wt. %, and wherein a weak organic acidis present in an amount of 5*10⁻⁶-0.1 Mole acid/g magnetic particle,

In an example of the present dispersion it comprises additives, such asa thickener in a concentration of 20-80 wt. % relative to the totalweight of the dispersion, such as a polymer, an oligomer, such as apolysaccharide, a starch, an elastomer, silica, a grease, andcombinations thereof.

Examples of further additives are quaternary ammonium compounds,quaternary ammonium sulphates, tetra-alkyl ammonium sulphate, andantioxidants, such as di-ethyl-hydroxyl amine. For fluids furtheradditives considered and used are antioxidants, such asN,N-hexylmethylamine; viscosity modifiers, such as clays, such asBentonite, silica nanoparticles, cellulose. For thickened fluids furtheradditives considered and used are elastomers, such as silicon rubber,styrene-butadiene-styrene thermoplastic elastomer; and viscositymodifiers such as silica particles.

Stabilizers may be included into the present liquids as well. Alsosurfactants may be included, such as poly isobutenyl succinic anhydride,and dodecyl benzene sulphonic acid.

In an example of the present dispersion it comprises one or more ofwater, an apolar liquid, such as oil, such as a perfluorinated oil, e.g.a polyalphaolefin oil, such as dioctyl sebacate a polar liquid, such asan alcohol, a weak acid, an aromatic, and an ionic liquid. Examples ofionic liquids are [HMIM][TBF]: 1-Hexyl-3-methylimidazoliumbis(trifluoromethylsulphonyl)imide and [BMIM][DC]:butyl-3-methylimidazolium dicyanamide. The above liquids are alsoreferred to as carrier liquids.

In hydrophilic ionic liquids, the particles as described in example 1can be used. For hydrophobic ionic liquids the best results are obtainedwhen the particles have interactions with ionic liquid functionalizedwith a carboxylic acid, an oxysilane or other group interacting group.Preferably the ionic liquid is similar to the functionalized ionicliquid.

In an example of the present dispersion the magnetic particles have anaverage size of 2 nm-10 μm, preferably from 3 nm-200 nm, more preferablyfrom 5 nm-100 nm, such as 10 nm. The particle size can be controlled atleast by concentrations of the iron salts, addition rate of the base andamount of the base in the coprecipitation method used in the synthesisof particles.

A specific surface area of the preferred present particles is from 6-600m²/g particle, more preferably from 20-100 m²/g particle, such as from40-60 m²/g particle.

In an example of the present dispersion the magnetic particles are oneor more of ferromagnetic particles, anti-ferromagnetic particles,ferrimagnetic particles, synthetic magnetic particles, paramagnetic,superparamagnetic, such as particles comprising one or more of Fe, Co,Ni, Gd, Dy, Mn, Nd, Sm, and preferably one or more of 0, B, C, N, suchas iron oxide, such as ferrite, such as magnetite, and maghemite.

In a third aspect the present invention relates to a use of a highdensity dispersion according to claim 12.

In a fourth aspect the present invention relates to an oil, ionicliquid, or water based fluid, that is having oil, ionic liquid, or wateras a (primary liquid) comprising the present dispersion, according toclaim 13.

In an example the present dispersion, specifically a magnetic fluidthereof is used functionalized with at least one poly-electrolyte, eachpolyelectrolyte producing a number of ions, for osmosis. In afresh-water compartment it is added to increase an osmotic pressurethereof. The pressure needed to be applied on e.g. saline water or seawater decreases. As such the process becomes much cheaper. It ispreferred to add an amount of magnetic fluid such that an osmoticpressure exceeds that of a second compartment comprising saline or seawater, that is e.g. higher than 2700 kPa (27 bar). The process ofosmosis continues until pressures in both compartments are substantiallyequal. At that moment a large amount of water will have been separatedfrom the saline water by osmosis, without a need to apply a pressure, orat the most a low pressure. The present magnetic fluid is e.g. thenseparated from the fresh water obtained, such as by application of amagnetic field. The compartment may then be emptied. The magnetic fluidcan be reused and the process can start again.

In the above example it is preferred to apply the present magnetic fluidin a relatively high amount or density such that at a relatively highosmotic pressure is obtained. Having a high density dispersion lessdispersion is needed. A preferred osmotic pressure is from 4-6 MPa(40-60 bar).

In an example the present dispersion, specifically a magnetic fluidthereof is used as a sensor: As an example a temperature sensor usingmagnetic particles dispersed in a gel. If melting temperature of thedispersion is surpassed, its conductivity decreases and can be detectedeasily. This change on conductivity when melting temperature is reachedallows using magnetic particles dispersed in a gel as a temperaturesensor at different applications, such as transport of sensitiveproducts that cannot reach temperatures above a certain limit.

In an example the present dispersion, specifically a magnetic fluidthereof is used as a sealant: As an example a dynamic sealing using amagnetic fluid. In several equipment containing a rotating axle, thereare two or more different environments that need to be hermeticallyseparated by sealing the hole through which the axle passes. This isachieved by placing a magnetic fluid in a groove in the shaft and theuse of a magnetic field to keep in place the magnetic fluid and thenavoid the displacement of any impurity from one environment to theother. For example, the hard disks of computers, that need to work inhermetic conditions to avoid the interaction of their internal part withany grain of powder or smoke present in the external environment.

In an example the present dispersion, specifically a magnetic fluidthereof is used as a heat transfer fluid: As an example using a magneticfluid for cooling of micro-system based on thermomagnetic convection: amagnet acts as a pump (with no moving parts, any power or maintenance)and the magnetic fluid acts as the coolant. Cool magnetic fluid is drawnto the part that requires cooling, it warms up and becomes less magneticand fresh cool magnetic fluid replaces it. This cycle may continueindefinitely.

In an example the present dispersion, specifically a magnetic fluidthereof is used as a damping fluid: Magnetic fluids are utilized in thefabrication of diverse applications such as brakes and high-performanceshock absorbers for civil engineering (bridges and buildings),industrial and household appliances (washing machines and fitnessequipment), medical appliances (prosthetic limbs) and automotiveindustry (cars and trucks). For instance a magnetic fluid provided withan electronic control unit for the car suspension that willautomatically adjust the properties of the magnetic fluid depending onthe current road conditions and driving manner to provide a highlysecure and comfortable driving.

In an example the present dispersion, specifically a magnetic fluidthereof is used in magnetic separation processes: Magnetic fluids can beused for the separation of magnetic from non-magnetic materials and forthe separation of materials by their density. For instance whenrecycling non-ferrous metals or plastics in waste or separating diamondfrom gangue material in the mining industry. Using a magnetic fieldcreates a magnetic pressure distribution in the magnetic fluid thatcauses the magnetic fluid to act as if it has a variable density thatchanges with height. Magnetic materials are attracted to the regions ofstrongest magnetic field, whereas nonmagnetic materials are displaced tothe regions of low magnetic field with matching effective density.

In an example the present dispersion, specifically a magnetic fluidthereof is used in a magnetic drug targeting application: As an examplea chemical drug bounded to a magnetic fluid is injected in a cancertumor and it is kept in the tumor by the use of a magnetic field for aspecific amount of time, for example an hour. The result of thistreatment is that the chemical drug has a very intense action and theamount of drug that is needed is much lower than the amount needed if itwas dispersed in the entire body. When the treatment is finished,magnetic field is turned off and the drug disperses in the body, but asthe amount of drug used for the treatment is really low, it has no sideeffects on the patient health.

In an example the present dispersion, specifically a magnetic fluidthereof is used as magnetic lubricant in lubrication processes: Magneticfluids mixed with an additive, such as grease, can be used to improvelubrication of dynamic machine elements, such as bearings, bypositioning the magnetic fluid to specific places in the machine elementapplying a magnetic field.

In a fifth aspect the present invention relates to a functionalizedmagnetic particle or magnetic dispersion according to the invention,such as functionalized with a suitable chemical group for catalystretrieval, functionalized with an active catalyst, functionalized with apolyelectrolyte, functionalized with a polymer, and functionalized witha reactive chemical group.

Functional groups considered are e.g. a carboxylic acid and anoxysilane, such as methoxysilane or ethoxysilane. These groups cancatalyze various reactions, such as depolymerization of polymers, suchas poly(ethylene terephthalate) (PET). PET can be depolymerized byfunctionalized magnetic particles into oligomers and mainly a monomer inorder to re-polymerize the latter and to re-obtain high quality PET forfurther use. Functionalized magnetic particles can be recovered at theend of the depolymerization process by the application of a magneticfield and be reused.

In an example magnetic dispersion are functionalized with at least onepolyelectrolyte, each polyelectrolyte producing a number of ions, forosmosis process, such as process to obtain fresh water from saline wateror sea water. Functionalized magnetic dispersion can be removed from thewater by the application of a magnetic field and be reused.

In an example magnetic dispersion are functionalized with a blockcopolymer, which is made up of blocks of different polymerized monomers,for increasing stability of magnetic particles present in a dispersionfor further uses of magnetic fluid thereof.

In an example magnetic dispersion mixed with an additive, such asgrease, can be functionalized with a reactive chemical group to be usedas reactive magnetic lubricant in certain additional applicationstogether with improving lubrication of dynamic machine elements, such asbearings.

The invention is further detailed by the accompanying examples, whichare exemplary and explanatory of nature and are not limiting the scopeof the invention. To the person skilled in the art it may be clear thatmany variants, being obvious or not, may be conceivable falling withinthe scope of protection, defined by the present claims.

SUMMARY OF FIGURES

FIG. 1A-H show general chemical structures.

DETAILED DESCRIPTION OF FIGURES

FIG. 1 shows general chemical structures of examples of a functionalizedweak organic acid according to the invention. FIG. 1A shows animidazolium based functional acid, FIG. 1B a piperidinium basedfunctional acid, FIG. 1C a pyridinium based functional acid, FIG. 1D apyrrolidinium based functional acid, FIG. 1E a sulfonium basedfunctional acid, FIG. 1F an ammonium based functional acid, and FIG. 1Gan phosphonium based functional acid. FIG. 1H shows functional group R₁,whereas FIG. 1I shows functional group R₂. Functional groups R₃ and R₄may be selected from H, CH₃ and R₁ and R₂. Functional groups R₁-R₄ areselected independently and may be (partly) the same, or not. The sidegroup having m or o carbon atoms may be branched, whereas the side grouphaving n carbon atoms is preferably straight.

Examples

The invention although described in detailed explanatory context may bebest understood in conjunction with the accompanying examples.

Experimental Procedure to Obtain Water-Based Magnetic Fluid Stabilizedwith Citric Acid

LIST OF CHEMICALS USED

Iron(III)chloride hexahydrate FeCl₃•6H₂O Iron(II)chloride tetrahydrateFeCl₂•4H₂O Citric acid monohydrate C₆H₈O₇•1H₂O Ammonium hydroxide NH₄OHAcetone (CH₃)₂CO Demiwater H₂O

1 Procedure

a) Synthesis of Particles (Using a Coprecipitation Method at RoomTemperature):

43.25 g of FeCl₃.6H₂O, 15.9 g of FeCl₂.4H₂O and surplus of demiwaterwere mixed. When after iron chlorides were dissolved, ammonium hydroxidewas added over 15 seconds. The obtained dispersion was stirred for 5minutes.

b) Stabilization with Citric Acid (at Room Temperature) and WashingSteps with Water and Acetone:

Thereafter, 50.43 g of citric acid were added. When the citric acid wasadded, the dispersion was stirred for 5 minutes, after which the beakerwas placed on a Neodymium magnet for 5 minutes. The clear supernatantwas decanted and separated from magnetic particles. Then, water wasadded to magnetic particles and stirred for 5 minutes until no moreaggregates were remaining.

When no more aggregates were visible, acetone was added and was placedon the magnet for 6 minutes. The dark yellow supernatant was removed andthe particles were redispersed in sufficient water by stirring it untilno more aggregates were visible. Then, acetone was added to the obtaineddispersion and was placed on the magnet for 5 minutes. The clear yellowsupernatant formed was decanted and only particles which remained on themagnet were redispersed in water and stirred until no more aggregateswere visible. After, acetone was added to the dispersion and placed onthe magnet for 2 minutes. The supernatant was decanted. Finally, a smallvolume of water was added until the particles were redispersed and ahomogeneous magnetic fluid was obtained (Small volume does not mean theminimum volume of water to redisperse the particles as it is possible toincrease the concentration of particles afterwards (see section c))).

c) Increase of the Magnetic Fluid Concentration of Particles:

After obtaining the magnetic fluid, the concentration of particles canbe increased until the desired value by leaving magnetic fluid in anopened vessel at room temperature overnight or at a temperature between40 and 60° C. for several hours.

Oil-Based Magnetic Fluid Production Procedure

List of Chemicals Used:

Iron(III)chloride (40% v/v) FeCl₃ Iron(II)sulphate heptahydrateFeSO₄•7H₂O Oleic Acid C₁₈H₃₄O₂ Ammonium hydroxide NH₄OH Acetone (CH₃)₂CODemiwater H₂O

1 Procedure

a) Synthesis of Particles (Using a Prior Art Coprecipitation Method atRoom Temperature):

17 g FeSO₄.7H₂O, 36 g of FeCl₃ and demiwater were mixed. When after ironchloride and iron sulphate were dissolved, ammonium hydroxide was addedover 15 seconds. The obtained dispersion was stirred for 5 minutes.

b) Stabilization with Oleic Acid and Washing Steps with Water andAcetone:

Thereafter, 20 g of oleic acid was added. When the oleic acid was added,the dispersion was stirred for 15 minutes. After which the beaker wasplaced on a Neodymium magnet for 5 minutes. The clear supernatant wasdecanted and separated from magnetic particles. Then, water was added tomagnetic particles and stirred for 5 minutes until no more aggregateswere remaining.

When no more aggregates were visible, acetone was added and was placedon the magnet for 6 minutes. The supernatant was removed and theparticles were redispersed in sufficient water by stirring it until nomore aggregates were visible. Then, acetone was added to the obtaineddispersion and was placed on the magnet for 5 minutes. The supernatantformed was decanted and only particles which remained on the magnet wereredispersed in water and stirred until no more aggregates were visible.After, acetone was added to the dispersion and placed on the magnet for2 minutes. The supernatant was decanted. Finally, a small volume ofoil-based carrier was added until the particles were redispersed and ahomogeneous magnetic fluid was obtained.

In the tables 1-2 below some further details of the present dispersionsare given, specifically for water and oil based ferrofluids.

TABLE 1 Volume and weight percentages of present water based ferrofluiddispersions. Water based Ferrofluid V/V % obtained wt. % obtained 1548.3 20 57 25 63.4 30 69.1 40 77.6

TABLE 2 Volume and weight percentages of present oil based ferrofluiddispersions. Oil based ferrofluids V/V % obtained wt. % obtained 15 50.220 58.9 25 65.6 30 71.3 40 79.3

It should be appreciated that for commercial application it may bepreferable to use one or more variations of the present system, whichwould similar be to the ones disclosed in the present application andare within the spirit of the invention.

What is claimed is:
 1. High density dispersion comprising magneticparticles, obtained by a method comprising the steps of providing adispersion with magnetic particles, directly thereafter providing a weakorganic acid, or salt thereof, in an amount of 5*10⁻⁶-0.1 Mole acid/gmagnetic particle, and interacting the weak organic acid and magneticparticles, maintaining the pH at a basic value, wherein after providingthe weak organic acid the dispersion is washed one to four times, andwherein a washed dispersion is re-dispersed in a solvent, and whereinthe dispersion is concentrated-to at least 15% v/v, wherein the magneticparticles are present in an amount >15 vol. % relative to a total volumeof the dispersion, and in an amount >45 wt. % relative to a total weightof the dispersion, and wherein a weak organic acid is present in anamount of 5*10⁻⁶-0.1 Mole acid/g magnetic particle on the magneticparticle.
 2. High density dispersion according to claim 1, comprising acombination of magnetic particles.
 3. High density dispersion accordingto claim 1, further comprising additives, such as a thickener in aconcentration of 20-80 wt. % relative to the total weight of thedispersion, such as a polymer, an oligomer, such as a polysaccharide, astarch, a gum, silica, a grease, an elastomer, and combinations thereof.4. High density dispersion according to claim 1, comprising one or moreof water, an apolar liquid, such as oil, such as a perfluorinated oil,such as a polyalphaolefin oil, a polar liquid, such as an alcohol, aweak acid, an aromatic, and an ionic liquid, and optional furtheradditives.
 5. High density dispersion according to claim 1, wherein themagnetic particles have an average size of 2 nm-10 μm, and/or whereinthe magnetic particles are one or more of ferromagnetic particles,anti-ferromagnetic particles, ferrimagnetic particles, syntheticmagnetic particles, paramagnetic particles, superparamagnetic particles,such as particles comprising one or more of Fe, Co, Ni, Gd, Dy, Mn, Nd,Sm, and preferably one or more of O, B, C, N, such as iron oxide, suchas ferrite, such as magnetite, and maghemite.
 6. High density dispersionaccording to claim 1, wherein the weak acid is a carboxylic acidcomprising 4-20 carbon atoms, for water based fluids and hydrophilicionic liquids comprising 5-10 carbon atoms, for oil based fluids andnon-hydrophilic ionic liquids 12-20 carbon atoms.
 7. Use of a highdensity dispersion according to claim 1, in a biological application, abiomedical application, for molecular diagnosis, as a magnetic targetingagent, as a coupling agent, for dry targeting, for analyzing samples,such as blood samples, as a sealant, especially a high pressuredifference sealant, a low vapor pressure sealant, a high temperaturesealant, a high magnetic field sealant, as a fast switch, as a gel, as asensor, such as a temperature sensor, an electrical conductivity sensor,an electrical sensor, and a strain sensor, and a multifunctional sensor,as an indicator, as a damper, as a shock absorber, as an actuator, as anelectrical resistor, in a magnetic fluid, for fast switching of sealingproperties, for osmosis, such as for obtaining fresh water from partlyor fully saline water, such as from seawater, for water treatment, forwater cleaning, for obtaining long term stability of the dispersion, asa high ionic strength dispersion, as a catalyst retriever, for magneticseparation of materials with different densities and/or magnetic fromnon-magnetic materials, in response to an external stimulus, such as oneor more of magnetic field, stress, electric field, and temperature, forchanging properties thereof, such as rheological properties, electricalproperties, melting temperature, as a lubricant, as a reactive magneticlubricant, and combinations thereof.
 8. Oil, an ionic liquid, or waterbased fluid, such as a ferrofluid, comprising a dispersion according toclaim
 1. 9. Functionalized magnetic particle or magnetic dispersionaccording to claim 1, such as functionalized with a suitable chemicalgroup for catalyst retrieval, functionalized with an active catalyst,functionalized with a polyelectrolyte, functionalized with a polymer,and functionalized with a reactive chemical group.
 10. Method of forminga high density dispersion according to claim 1, comprising the steps of:providing a dispersion with magnetic particles, directly thereafterproviding a weak organic acid, or salt thereof, in an amount of5*10⁻⁶-0.1 Mole acid/g magnetic particle, such as a short or mediumchain carboxylic acid, and interacting the weak organic acid andmagnetic particles, wherein interacting takes place until largelycompleted, during a period of less than 5 minutes, and whereininteraction is supported by mixing, maintaining the pH at a basic value,preferably between 7.3 and 10, wherein after providing the weak organicacid the dispersion is washed one to four times, and wherein a washeddispersion is re-dispersed in a solvent, and wherein the dispersion isconcentrated to at least 15% v/v relative to a total volume of thedispersion and to >45 wt. % relative to a total weight of thedispersion.
 11. Method according to claim 10, wherein the magneticparticles are one or more of ferromagnetic particles, anti-ferromagneticparticles, ferrimagnetic particles, synthetic magnetic particles,paramagnetic particles, superparamagnetic particles, such as particlescomprising one or more of Fe, Co, Ni, Gd, Dy, Mn, Nd, Sm, and preferablyone or more of O, B, C, N, such as iron oxide, such as ferrite, such asmagnetite, and maghemite.
 12. Method according to claim 10, wherein theweak acid is a carboxylic acid, the carboxylic acid comprises a 4-20carbon atoms, for water based fluids 5-10 carbon atoms, for oil basedfluids 12-20 carbon atoms, and/or wherein the carboxylic acid comprises1-4 carboxylate groups, such as 2-3 carboxylate groups, such as citricacid, and a polycarboxylic acid, or wherein the weak acid is afunctionalized acid.
 13. Method according to claim 10, wherein thedispersion with magnetic particles is provided at a temperature between15° C.-95° C., wherein the weak organic acid is provided in an amount of10⁻⁴-0.01 Mole acid/g magnetic particle, and/or wherein the method isperformed in air.
 14. Method according to claim 10, wherein afterwashing the dispersion is finally dispersed in water.
 15. Methodaccording to claim 10, wherein the weak organic acid is provided at atemperature between 15° C.-95° C.